For purposes of clarity and consistency, the following terms as used throughout this text and the appended claims should be interpreted as follows:                The phrase “substantially planar” should be construed as referring to a substrate in the (approximate) form of a sheet, plate, leaf, wafer, platen, etc. Such a substrate will generally be (substantially) flat in form, and present two opposed major surfaces separated by a relatively thin intervening “sidewall”.        The phrase “semiconductor substrate” should be broadly interpreted as encompassing any substrate on which a semiconductor device or other integrated device is manufactured. Such substrates may, for example, comprise silicon or germanium wafers (of various diameters), and/or wafers of compound substances such as InAs, InSb, InP, GaSb, GaP or GaAs. The term also encompasses non-semiconductor materials (such as sapphire) on which one or more layers of semiconductor material have been deposited, e.g. as in the manufacture of LEDs. The semiconductor device or other integrated device concerned may, for example, be an integrated circuit, (passive) electronic component, opto-electronic component, biological chip, MEMS device, etc. Such devices will generally be manufactured in large numbers on a given substrate, and will typically be laid out in a matrix arrangement on at least one of said major surfaces.        The term “scribelane” (also sometimes referred to as a “dicing street”) should be interpreted as referring to a path (tract) extending along a major surface of a substrate, along which path the substrate is to be scribed. In the case of a semiconductor substrate, a scribelane will generally extend between neighboring/adjacent/opposed rows of integrated devices on the substrate, and it represents a street along which the substrate is to be “diced” so as to allow (ultimate) separation of the devices in question. Such a procedure is often referred to as “singulation”. It should be noted that scribelanes on the target surface may be arranged in regular and/or non-regular (repetitive) configurations. For example, some wafers may comprise a regular matrix of identical integrated devices separated from one another by scribelanes forming a regular orthogonal network. On the other hand, other wafers may comprise devices of different sizes, and/or located at non-regular pitches with respect to one another, implying a correspondingly irregular configuration of scribelanes. The arrangement of such scribelanes does not necessarily have to be orthogonal, e.g. it might be trigonal or hexagonal in nature.        The phrase “radiatively scribing” refers to a procedure whereby one or more focused light beams are used in scanning motion to effect a permanent physical change in the material occurring along a scribelane, e.g. by ablating it, cracking it or shocking it, for instance. Such a procedure may be used to sever/singulate the substrate through its full thickness. Alternatively, it may be used to create a so-called “groove” (gouge, furrow, channel), which is a form of scribe that does not penetrate through the full thickness of the substrate in which it is created, i.e. creation of the groove does not directly cause severance of the substrate (in the Z direction). Substrate singulation involving such grooving is thus necessarily a multi-step approach, whereby one or more follow-up procedures are used to finish off the severing process, such as additional radiative scribing, mechanical sawing/cutting, etc. along the previously created groove.        The term “light” should be broadly construed as referring to electromagnetic radiation at visible, infrared and/or ultraviolet wavelengths.        The phrase “projection system” should be interpreted as referring to an optical system that is capable of focusing an incoming light beam to a given point. Such a system will generally comprise one or more lenses and/or mirrors.These points will be discussed in more detail below.        
Radiative scribing (also known as laser scribing) using an array of several light beams is a well-known technique in the art of wafer singulation. In early techniques of this type, the employed array of light beams comprised a linear group of beams that was oriented along the X direction, parallel to the scribing motion: such a set-up is described, for example, in U.S. Pat. Nos. 5,922,224 and 7,947,920, and United States Patent Application US 2013/0217153 A1. Later techniques employed a linear group of beams oriented along the Y direction, perpendicular to the scribing motion, as described, for example, in US 2005/0109953 A1. Recently, a hybrid approach employing a two-dimensional array of light beams (distributed parallel to both X and Y) has also been developed, as set forth in co-pending, non-published European patent application EP 13167717, which has a common inventor/assignee with the current application. All of the patent references cited in this paragraph are incorporated herein by reference.
In all of the above techniques, the employed illuminator must have some means of generating plural light beams. One way to provide such beams is to use a plurality of lasers, e.g. as set forth in the above-mentioned document US 2005/0109953 A1; however, in general, such an approach is currently considered to be relatively unattractive, inter alia because of the relatively large unit price for suitable lasers (such as Nd:YAG or doped fiber lasers). Another approach is to split the output from a single laser and convert it into plural beams, using some sort of beam splitter arrangement, e.g. as set forth in WO 2009/102002 A1; however, such an approach can become unattractively complicated when the produced beam array is required to comprise more than two beams, and/or as regards adjustability of the spatial configuration of beams in the array. Yet another approach uses a Diffractive Optical Element (DOE) to diffractively split an input beam into a plurality of output beams, in a given spatial arrangement. Such a method is used, for example, in the above-mentioned U.S. Pat. Nos. 5,922,224, 7,947,920, US 2013/0217153 A1 and EP 13167717. DOEs can be tailor-made to produce a desired beam array, can be stacked in series if desired so as to produce hybrid arrays, and can easily be mounted in an exchanging mechanism (such as a carrousel) to allow the beam configuration to be varied/adjusted. In addition to the above-mentioned (and possible alternative) means for producing multiple beams, an illuminator may also comprise further components, such as imaging/focusing elements, filters, etc., for performing specific processing operations on said beams.
An illuminator as set forth above may disadvantageously produce a number of unwanted “parasitic” light beams in addition to the desired array of “main” light beams. Such parasitic beams can be produced by different mechanisms. For example, when beam sub-division is performed using a DOE, higher diffraction orders than the orders in the main beam array can produce (relatively weak) satellite beams outside the perimeter of the main beam array. Alternatively/additionally, multiple reflections in optical components such as beam splitters, plan-parallel plates, etc. can produce unwanted ancillary beams. Such parasitic beams can substantially disrupt/erode the accuracy of a radiative scribing procedure if they fall onto “device regions” of a semiconductor substrate located outside a scribelane, where the heat they produce (upon impingement on the substrate) may damage delicate devices, e.g. by causing effects such as burning, melting, cracking, discoloration, delamination or a change in other physical/chemical properties (such as dielectric constant, impedance, crystalline phase, etc.). These effects are generally exacerbated in situations where the employed beam array is adjustable, since variable main beam configurations (involving adjustable numbers/spacings of main beams, shape of array, etc.) can produce relatively elaborate/unpredictable distributions of parasitic light beams.
Another issue of note involves illuminators in which multiple beams are generated by beam sub-division, which—as set forth above—can involve a relatively complex mechanism of nested beam splitters or exchangeable DOEs. A situation may arise in which a user of a radiative scribing apparatus has carefully adjusted the illuminator to produce a beam array of a first shape/geometry (e.g. using a specific DOE, or a given beam splitter configuration) for the purposes of scribing a relatively large first batch of substrates, but would also like to scribe a relatively small or urgent second batch of different substrates using a different array shape/geometry. In current apparatus, regardless of the size or importance of a batch, the same (painstaking) mechanism of beam array adjustment must be used—which can cause additional expense (e.g. if a DOE has to be tailor made) and/or can take additional time/effort (e.g. to adjust beam sub-dividing components, or in awaiting manufacture/delivery of a specified DOE).